This invention relates to a method and apparatus for supervising the injection stroke of an injection molding machine, and more particularly to a method and apparatus for supervising the injection stroke wherein at least one detector is mounted on an injection molding machine, or a portion thereof, for example, a metal mold, and sampled pressures or speeds detected by the detector are compared with a reference value so as to judge whether the injection speed or pressure (hereinafter termed an injection parameter or factor) is optimum or not.
According to most of prior art supervisory apparatus, a detected value or pressure in which time is used as a variable is checked as to whether the detected value is within a permissible range or not at a preset time. To judge the performance of the filling state during a filling or injection stroke it has been necessary to judge whether the variation curve of the detected value coincides with the curve of a reference state or not, so that it is not sufficient to judge the injection parameter only at a set point.
As a solution of this problem the applicant has proposed a method of judging the deviation of the variation curve of the detected value from the reference curve by comparing detected values at a plurality of sampling points with a permissible value as disclosed in Japanese Laid Open Patent Specification No. 156663/1980. When carrying out this method into practice, it was noted that there are the following problems. Because, since the filling state varies variously it is impossible to select the sampling period to be commensurate with a specific state. For example, the performance at the build up portion of the filling stroke and that near the end of packing constitute important factors having a great influence upon the quality of the molded product. Since such factors change rapidly unless the sampling is made quickly, it is impossible to correctly know the filling states at the build up portion and the packing completion portion. However, at points intermediate of these portions, as the variation is slow, the sampling speed may be low.
When a sampling pattern is determined for the aforementioned pattern of the filling stroke, and where the injection speed or pressure varies greatly in the intermediate region, the sampling period would become large so that it becomes difficult to correctly grasp the rapid variation in the intermediate region.
In addition, where the capacity of a memory device is selected by taking into consideration such other factors as too short sampling period, difference in the sampling numbers caused by the difference in the filling time, and a molding requiring a long molding time, it is obliged to use a memory device having a very large capacity.